A general carbohydrate partitioning principle in maize holds that photoassimilate moves preferentially from a leaf to the nearest metabolically active tissue (i.e., source to sink). Edmeades, et al., (1979) Can J. Plant Sci. 59:577-584. Thus, of various leaves, the ear-leaf supplies the greatest percentage of assimilate to the developing ear of the plant.
It is also well understood that canopy formation is a critical process in crop development, as it sets the stage for successful seed set and seed filling. The importance of solar radiation capture to yield in plants, including maize, is well established. In the United States Cornbelt, plant populations have consistently increased over the past 50 years. See, e.g., Iowa State University: Corn Planting Guide (September 2001). As plant populations increase, the ear-leaf receives less incident solar radiation and radiation which is of reduced quality, due to the development of the canopy. As a result, at high plant populations, the primary source leaf for developing ears is situated in an inferior light environment. Egharevba, et al., demonstrated that the ear-leaf of plants grown at high densities receives decreased quantity and a reduced quality of solar radiation due to its position in the canopy wherein light transmission is decreased. Egharevba, et al., Proc. of Physiol. Prog. Formulation Workshop IITA, 18-20, 1975.
There is also a documented photosynthetic compensatory response to defoliation in maize. Allison, et al., (1966) Ann. Bot. N.S. 30:365-381. Increased photosynthetic rates have also been demonstrated in response to partial defoliation of rangeland plants, where remaining leaves demonstrate a compensatory increase in photosynthetic rates. Briske, et al., (1995) Wildland plants: physiological ecology and developmental morphology, 635-710. It is hypothesized that the compensatory increase in photosynthetic rates is an evolved survival mechanism against grazing by herbivores.
Interestingly, the literature available generally reports detrimental effects of defoliation on maize yields. See, e.g., Hicks, et al., (1977) Agronomy J. 69:387; Tollenaar, et al., (1978) Can. J. Plant Sci. 58:207; Crookston, et al., (1978) Crop Sci. 18:485; Johnson, (1978) Agronomy J. 70:995 and Hunter, et al., (1991) Crop Sci. 31:1309. There is literature recognizing the ability of total defoliation to improve aspects of maize seed quality, but seed size is reduced. U.S. Pat. No. 6,162,974. Additionally, Crookston, et al., reported that defoliation can enhance yield in short-season maize hybrids if leaf removal occurred at a very early growth stage, prior to flowering. However, leaf removal during silking consistently led to a reduction in maximum yield.
The general estimated percentage of maize grain yield loss as a result of defoliation has been studied at various growth stages. National Crop Insurance Services (Rev. 1984). See, Table 1. The growth stages in Table 1 were determined by counting leaves using the leaf over method, counting a leaf if it has emerged from the whorl and the leaf tip is starting to arch over.
Commercial data suggest that plant population densities can be increased to generate increased plant yields due to improved plant genetics and germplasm, biotechnology traits in seeds, seeding rates and plant placement. However, such studies typically test densities ranging from 23,000 to 43,000 seeds per acre and demonstrate a drop-off of increased yields above 38,000 seeds per acre. The testing of various seeding rates (seeds per acre) is not equivalent to the lower number representing the final stand or actual number of plants reaching maturity. Accordingly, the results represent actual plant density less than the referenced 23,000 to 43,000 seeds per acre.
These references do not show or address, however, the impact of defoliating in high density planting of crops. Accordingly, the development of defoliation methods to produce an unexpected increase in grain yield, average kernel mass and biomass yield would enable a variety of commercial uses. Increased grain yield is particularly important for fields planted at increased populations for purposes of inbred production. Increased kernel biomass production and biomass yield are particularly important and useful for generating increased dry mass for a variety of commercial purposes. Further, an increase of photosynthetic rates of plants further enhances the ability of such plants to produce such increased yields and kernel biomass for the same variety of commercial purposes. Further, planting in areas of increased plant density is also desirable to enable growers to greatly exceed industry standard limitations on density of planting in fields. Still further, the methods of increasing yields, kernel and biomass are also likely desirable for plants under drought stress, where a yield increase is hypothesized to result from a decrease in leaf surface area requiring ground water.
TABLE 1% Leaf DefoliationGrowth101520253035404550556065707580859095100Stage% Yield Loss   7 leaf0000001123445567899   8 leaf000001123455667891011   9 leaf00011223456677910111213  10 leaf000123456788991113141516  11 leaf001123567891011121416182022  12 leaf00123457910111315161820232628  13 leaf011234681011131517192225283134  14 leaf0123468101315172022252832364044  15 leaf1123579121517202326303438424651  16 leaf12346811141820232731364044495561  17 leaf23457913172124283237434853596572  18 leaf23579111519242833384450566269768419-21 leaf3468111418222732384351576471798796Tassel35791317212631364248556268758391100Silked3579121620242934394551586572808897Silks brown2468111518222731364147546066748190Pre-blister2357101316202428323743495460667381Blister2357101316192226303439455055606673Early milk234681114172024283236414550556066Milk12357912151821242832374145495459Late milk1234681012151821242832353824650Soft dough1122468101214172023262932353841Early dent00112357911131518212325272932Dent0001234678101214151719202123Late dent0000123456789101112131415Nearly0000000012345566778matureMature0000000000000000000
Commercial data suggest that plant population densities can be increased to generate increased plant yields due to improved plant genetics and germplasm, biotechnology traits in seeds, seeding rates and plant placement. However, such studies typically test densities ranging from 23,000 to 43,000 seeds per acre and demonstrate a drop-off of increased yields above 38,000 seeds per acre. The testing of various seeding rates (seeds per acre) is not equivalent to the lower number representing the final stand or actual number of plants reaching maturity. Accordingly, the results represent actual plant density less than the referenced 23,000 to 43,000 seeds per acre.
These references do not show or address, however, the impact of defoliating in high density planting of crops. Accordingly, the development of defoliation methods to produce an unexpected increase in grain yield, average kernel mass and biomass yield would enable a variety of commercial uses. Increased grain yield is particularly important for fields planted at increased populations for purposes of inbred production. Increased kernel biomass production and biomass yield are particularly important and useful for generating increased dry mass for a variety of commercial purposes. Further, an increase of photosynthetic rates of plants further enhances the ability of such plants to produce such increased yields and kernel biomass for the same variety of commercial purposes. Further, planting in areas of increased plant density is also desirable to enable growers to greatly exceed industry standard limitations on density of planting in fields. Still further, the methods of increasing yields, kernel and biomass are also likely desirable for plants under drought stress, where a yield increase is hypothesized to result from a decrease in leaf surface area requiring ground water.